Fuel cells were invented in 1839 by Sir William Grove. A fuel cell is an electrochemical device which directly combines a fuel and an oxidant such as hydrogen and oxygen to produce electricity and water. It has an anode and a cathode spanned by an electrolyte. Hydrogen is oxidized to hydrated protons on the anode with an accompanying release of electrons to form water, consuming electrons in the process. Electrons flow from the anode to the cathode through an external load, and the circuit is completed by ionic current transport through the electrolyte.
Fuel cells do not pollute the environment. They operate quietly, and high temperature fuel cells have a potential efficiency of ca. 80 percent. Virtually any natural or synthetic fuel from which hydrogen can be extracted--by steam reforming, for example--can be employed.
It is well-known that oxidation-reduction reactions are accompanied by the transfer of electrons from the reductant to the oxidant. In fuel cells, the oxidation reaction and reduction reactions take place at spatially separated electrodes. Each of these reactions is called a half-cell reaction. The anode is the site of the oxidation half-cell reaction. A reactant, referred to as the fuel, that is oxidizable with respect to some oxidant, is supplied to the anode and is there electrochemically oxidized. Oxidation of the fuel releases electrons to the anode.
The cathode is separated from the anode by a tile which is a mixture of electrolyte and an inert material which remains solid at the fuel cell operating temperature. The other half-cell reaction simultaneously takes place at the cathode. The oxidant, which is reducible with respect to the fuel, is supplied to the cathode and is there electrochemically reduced. This reaction takes up electrons from the cathode.
These two half-cell reactions result in the cathode having a deficiency of electrons and the anode an excess of electrons. Therefore, a current will flow in an external circuit connected between the anode and the cathode as long as fuel and oxidant are available at the anode and cathode and waste (or reaction) products are removed from the reaction sites.
Modern, state-of-the-art fuel cells typically employ molten carbonate electrolytes and operate at temperatures in the range of 500.degree.-750.degree. C. Such cells have an operating efficiency in the 40 to 80 percent range.
Molten carbonate (MCFC) fuel cells require porous electrodes which have a uniform microstructure and will remain stable over long periods of time (typically required to be at least 40,00-50,000 hours) at the high temperatures and in the corrosive environments in which they operate.
Also, MCFC and other fuel cells commonly have a porous, electrode compatible, metallic bubble barrier on the face of the fuel cell anode or cathode or both of these electrodes. If the electrolyte filled tile does not pose an adequate barrier to the fuel and oxidant or the tile cracks, the fuel and oxidant may mix. At a minimum, this may result in a voltage drop and/or a loss of fuel. In more severe cases, there may be service life shortening oxidation of the anode and/or the creation of a potential safety hazard. In fuel cells equipped with bubble barriers, the pores of those components fill up with the electrolyte and prevent mixing of the fuel and oxidant in circumstances less catastrophic than those which result in a blowout of the electrolyte. Bubble barriers are required to remain stable and serviceable for the same periods of time under the same operating conditions as the electrodes with which they are associated in a MCFC or other high temperature fuel cell.
The porous metal MCFC components discussed above--cathodes, anodes, and bubble barriers--are currently fabricated from very fine base metal powders such as the 2-3 micron carbonyl nickels. Alloying metals such as Cu, Al, Ti, etc. are commonly added to the base metal to improve critical properties of the component being fabricated. For example: a nickel anode alloyed with one of these metals can be internally oxidized to form a dispersion of the alloy metal oxide in the nickel matrix. This produces a creep resistant anode.
Fine alloy powders are generally costly to produce, and they are often not readily available in the marketplace.
Furthermore, alloyed metal components for high temperature fuel cells such as the state-of-the-art Ni-Cr anodes are made by either tape casting or by loose sintering a physical mixture of metal powders and heat treating the resulting shape at elevated temperatures to diffuse the alloying metal throughout the base metal. To obtain a uniform dispersion of the second phase requires prolonged time at temperature. This may result in an unacceptable decrease in the porosity of the metal component.
The use of pack cementation to fabricate components for high temperature fuel cells has also been proposed. In this process, a porous component fabricated from the selected base metal is imbedded in a cementation pack consisting of an activator salt, a master alloy containing the alloying metal to be added, and an inert filler. The electrode is prevented from sintering to, or being contaminated by, the pack via a costly, inert ZrO.sub.2 or Al.sub.2 O.sub.3 loaded, porous paper. The electrode and pack are placed in a container having a reducing environment and raised to elevated temperatures at which the master alloy, such as Ni-3Al, reacts with the activator salt, such as NaCl, forming metal halide vapors such as aluminum chlorides. The metal halide vapors deposit on the base metal surface, and inward diffusion of the alloying metal results in alloy formation with the base. This is an expensive, batch-type process which adds considerably to the cost of making the electrode or other fuel cell component. And, as yet, a simple method of ensuring the even distribution of the pack materials required for needed homogeneity in the component being fabricated has not been discovered.